This invention relates to solvents for ethylene copolymers. In one aspect, this invention relates to solvent blends for copolymers of ethylene and ethylenically unsaturated carboxylic acid. In another aspect, this invention pertains to solvent blends which will hold such copolymers in solution at a temperature of about 60.degree. C.
Copolymers of ethylene and ethylenically unsaturated carboxylic acid are well known, and a number of such copolymers are commercially available. Further, such copolymers are characterized, among other things, by their outstanding adhesion to a variety of metallic and non-metallic substrates, outstanding toughness and tensile strength. Because of these unique properties, such copolymers find commercial utility, among others, in packaging, building products, communications and transportation end use applications. It has been found that a most important property of such copolymers is their adhesive characteristics. Thus, such copolymers are used widely as adhesive for laminating two or more substrates into a laminate. In a dry film lamination technique, the copolymers are first converted into dry adhesive films then placed between two substrates to form an intimate bond under heat and pressure. In an extrusion lamination technique, a molten coating of copolymer is extruded into a nip of two substrates to form a laminate. In an adhesive lamination technique, a solution or an aqueous dispersion of copolymer is first coated unto a substrate using a roll coater. The solvent or water is then dried off and the dried, coated substrate comes in contact with another substrate under pressure to form a laminate. When an adhesive lamination technique is involved, it is important to have an effective solvent system for copolymers. More specifically, an effective solvent system is needed to dissolve the copolymer in an adhesive solution. Still another need is an effective solvent system capable of cleaning applicator rolls (e.g. gravure rolls) and other laminating equipment which handles copolymer solutions or dispersions at an elevated temperature.
Tachi et al. (Kogyo Kagaku Zasshi 73(2), 398-42 (1970)) discloses the solubility of ethylene/acrylic acid copolymers in organic solvents, including mixed solvents. Tetrahydrofuran and certain mixed solvents (e.g. cyclohexane and dioxane) can dissolve the copolymer at 60.degree. C. However, none of these single solvents or mixtures suggest the ternary solvent blends of the present invention. German OLS No. 2,814,030 discloses a solution of graft ethylene acrylic acid at elevated temperatures in organic solvents such as cyclohexane to prepare "suspensions" of fine (insoluble) particles at cooler temperatures. Cyclohexane at an elevated temperature is used. There is no suggestion of the ternary solvent blends of the present invention. U.S. Pat. No. 3,711,456 discloses a method for the preparation of ethylene carboxylic acid polymers having improved optical properties. Such polymers are obtained by intimately contacting a normally solid random copolymer with a "polyethylene" solvent at elevated temperature, and thereafter separating the solvent from the solid ethylene polymer. The low molecular weight crystalline, low acrylic acid portion of the ethylene carboxylic acid copolymer is selectively extracted at elevated temperatures. Only single solvents are used. U.S. Pat. No. 3,814,737 discloses the use xylene/acetic acid and xylene/isopropanol as solvent mixtures for ethylene carboxylic acid copolymers at elevated temperatures. The temperature involved exceeds 100.degree. C. Further, binary solvent mixtures are used. U.S. Pat. No. 3,554,957 discloses a solvent system for copolymers of ethylene and acrylic acid which will maintain the copolymer in solution at substantially normal room temperature. The solvent system comprises between about 55 and about 65 percent by weight toluene, based upon the total weight of the solvent blend, and between about 35 and about 45 percent by weight tetrahydrofuran, based upon the total weight of the solvent blend. The copolymer can be dissolved and held in solution at normal room temperature in amounts up to about 5 percent by weight based upon the total weight of the solution. However, the solvent system of this patent is ecologically "unclean."
Accordingly, it is an object of this invention to provide a solvent system for copolymers of ethylene and ethylenically unsaturated carboxylic acid which is effective at a temperature of about 60.degree. C. Another object of this invention is to provide a solvent system which is ecologically "clean." It is yet another object of this invention to provide a solution containing a dissolved copolymer of ethylene and ethylenically unsaturated carboxylic acid which can be applied or deposited at a temperature of 60.degree. C. Other objects and advantages of the invention will become apparent in the following description.